I have a solar set up with a charge controller and a single 12v deep cycle battery. After taking the set up on a trip once, we realized the battery had a much lower capacity than we calculated. We took it to a battery shop and they tested it, and confirmed it was defective. We got a new one from them, and they showed that their tester confirmed it was like-new. I thought my problems were solved.

I hooked it up to the solar set up. We have not had time to use our full load on it yet, but it seems like it is draining itself too much. Today was a sunny day, and the charge controller reported that the battery was fully charged (5 bars, I didn't look at the volts at that time). Before the sun even fully went down, the battery had dropped to 3 bars. I checked the voltage, it was around 13.2 volts.

I thought that maybe the charge controller or the inverter were draining from it, so I disconnected both of those. I checked the voltage again (10 mins later), and it had dropped to 13.17 V. I disconnected all wires from it, and it is just sitting in my garage now, and it is draining about 0.01-0.02 volts per 10 minutes.

This seems like *way* too much. Some quick googling showed that lead acid batteries drain 5% per month. Assuming that 5% is of total voltage, not usable voltage, that would mean (0.6 V per month). At the rate this is going, it will drain that in 6-12 hours.

Did I somehow get unlucky and get a bad battery again? Or is it that my charge controller is over charging the batteries and damaging them? Could that damage happen in just 2 days?

Not sure what you paid for that battery but I'd scrap the 12v and go with 6v golf cart batteries...... you'll pay around $230 for two of these but you'll have 12v with 230 amp hours connecting them in series.

A fully charged battery fresh off the charger will show 13.2v or so but that float charge will quickly drain off and within a few hours or overnight it will read ~12.7v which is considered a fully charged battery.

You may also want to check how much drain the invertor puts on the system when it is just humming along and not doing any work. It takes a certain amount of juice to excite things and the cooling fan takes a certain amount of juice.

I know on a bus I had that had a 110-vac video system in it the invertor could suck two 8D bus batteries down to below 11.0 volts within a matter of hours with nothing else turned on. It wouldn't go much below 11.5 volts. Once it got down that low the invertor would shut it self off to keep from over heating the invertor. It takes a lot more amps at 11.5 volts to make 110-vac than when the input volts are 12.7 volts.

Roach and cowlitz are both correct. You can't measure the voltage on a battery until it has been at rest for several hours. 12.7 volts is a full charge. And inverters do use phantom power when turned on and nothing in use. Not too much but on one battery it is significant.

The battery monitor made for your charge controller gives you a very inaccurate read on your batteries. You have to spend some money if you want an accurate reading. Expect to pay $150 or more to get a good battery monitor. It is an essential part of your system. The monitor you have will always leave you guessing. Like driving to work with a broken fuel gage after your teenager used the car the night before.

Also do not expect to drain your battery 50% or more on a regular basis and expect it to last more than a year. If you only use 20-25% and fully recharge in between it can last for 7-10 years. Staying at a state of discharge kills batteries. Keep it fully charged and it will last a long time.

Your battery is fine. Call your battery manufacturer and ask them for the approximate voltage readings correlated to percentage of battery remaining. Also find out what voltage you need to bulk charge, absorb, float, and equalize. Get a charge controller that you can manually input the correct settings on and a battery meter that you can plug custom values into. 12.7 is fully charged -- 13.3 is float in your case. Your meter is wrong.